During which process are flocs formed?

Given below are two statements with reference to the secondary treatment of sewage.
Statement $I$: The $BOD$ of effluent increases.
Statement $II$: Flocs of bacteria consume the major part of the organic matter present in the effluent.
In the light of the above statements,choose the most appropriate answer from the options given below.

In sewage treatment,which of the following removes floating debris?

How is primary sludge formed?

In sewage treatment,after the $BOD$ is reduced to the required level,the effluent is passed into which of the following?

What is the key difference between primary and secondary sewage treatment?